The formation of video images on a screen by the movement of a modulated laser beam in both an x and y direction is analogous to image formation by electron beam deflection in a cathode ray tube (CRT). It is also known to use a lens with such a CRT system to project an image on a distant screen.
The use of acousto-optic deflectors and modulators for video image projection has also been proposed as shown, for example, in U.S. Pat. No. 4,720,747 and in J. M. Rouvan et al. "Theoretical study of the acousto-optic diffraction of light waves by acoustic waves in Lithium Niobate Crystals," Applied Optics, Vol. 29, No. 9, pp. 312-1316, March 1990. Common to all previously disclosed display devices employing acoustic waves is that they form a line of pixel images inside the deflection cell and then focus that line on a distant screen. The acoustic deflection in those devices was limited to sweeping the line of pixels vertically on the screen. This was necessary because it is not possible to sweep a single-pixel beam in both x and y directions rapidly enough to form a full, flicker-free image.
The reason that this is so, is that in order to change the angle of acousto-optic deflection, one must replace the acoustic wave of one wave-length with another of a different wave-length throughout the deflection cell. This cannot be done in less time than it takes for the acoustic wave to travel across the cell, and that sets a minimum time for changing the deflection angle. The limit so set is not sufficiently small to accommodate the needs of both horizontal and vertical deflection in a standard, two-dimensional raster-scan display. It is, however, adequately small to accommodate the scanning in just the vertical direction.
In either embodiment, a key feature is that the image is formed in focus at all screen distances simultaneously. Thus no focusing lens is needed to create a sharp image, and arbitrarily non-flat screen surfaces may be used.